1. Field of the Invention
The present invention relates generally to a method for antenna beam pointing or orientation in a satellite mobile communications system, and more specifically to such a method which constantly or intermittently compensates for an output of a rate gyro while automatically tracking the satellite, and obviates the need for a highly precise, expensive rate gyro and for a constant temperature chamber therefor (for example).
2. Description of the Prior Art
Before turning to the present invention it is deemed advantageous to discuss a known antenna beam pointing (stationary satellite tracking) technique with reference to FIGS. 1 to 4.
FIG. 1 is a sketch schematically illustrating a satellite mobile communications system wherein there is shown a stationary satellite 10 through which a plurality of automobiles 12, 14 and a ground station 16, are able to communicate with one other. As shown, the automobiles 12, 14 are respectively equipped with antennas 12', 14', while the earth station is provided with a parabola antenna 16'.
FIG. 2 illustrates a phased array type land mobile antenna system 18 which corresponds to each of the antennas 12' and 14' shown in FIG. 1. The antenna system 18 is comprised of a dielectric plate 20, which is mounted on a rotatable pedestal 22 and which carries four antenna elements 24a-24d in this case. Each of the antenna elements 22a-22d is a spiral form microstrip line. The dielectric plate 20 and the rotatable pedestal 22 are covered by a radome 26. The arrangement shown in FIG. 2 is well known in the art.
FIG. 3 shows schematically a fan beam 28 formed by a phased array antenna 30 mounted on the roof of an automobile 32. This antenna features a construction of the nature shown in FIG. 2.
Merely by way of example, the fan beam 28 has a half power beam width of about 20.degree. in azimuth (AZ) plane and about 80.degree. in elevation plane. This, as will be understood, randers the tracking of the stationary satellite in elevation plane unnecessary.
FIG. 4 is a block diagram showing a known antenna beam orienting system, which includes a phased array antenna 40 of the nature shown in FIG. 2. Accordingly, the numerals 24a-24d of FIG. 2 are also used to denote like elements of the antenna 40.
The beam direction of the antenna 40 can be changed in two azimuths by switching four phase shifters 42a-42d in order to specify the antenna azimuth relative to the satellite position. The switching of the phase shifters 42a-42d is performed in accordance with a predetermined repetition frequency of a reference signal applied thereto from a reference oscillator 44 via a bias tee 46 and a rotary joint 48. The bias tee 46 is a unit which includes an inductor L and a capacitor C. The bias tee 46 steers the reference signal from the reference oscillator 44 toward the rotary joint 48, while directing an RF (Radio Frequency) signal from the rotary joint 48 to a transceiver 50. On the other hand, the rotary joint 48 establishes an electrical contact between a rotating cable attached to the rotatable antenna and the fixed cable coupled to the bias tee 46.
The transceiver 50 includes a diplexer 52, a modem 54, etc. Transceivers which are utilized in satellite communications system are well known in the art and hence the detailed description will be omitted for the sake of brevity. Although not shown in FIG. 4, the modem 54 includes a receive signal level detector which is supplied with an output of an AGC (Automatic Gain Control) amplifier provided in an IF (Intermediate Frequency) stage. A coherent detector 56 receives the above-mentioned receive signal level (RSL) and synchronously detect the antenna angular position error (APE) with the aid of the reference signal applied from the oscillator 44. The output of the coherent detector 56 (viz., the angular position error) is applied to a switch 58.
As shown, a rate gyro 60 is provided and outputs a signal indicative of the yaw rate of the vehicle around the azimuth axis thereof. The voltage output of the rate gyro 60 is applied to the switch 58.
A comparator 62 is supplied with the above-mentioned receive signal level (RSL) at one input of a comparator 62 and receives a threshold at the other input thereof. In the event that the receive signal level RSL is higher than the threshold, the output of comparator 62 (viz., switch control signal (SCS)) allows the switch 58 to apply the antenna angular position error (APE) derived from the coherent detector 56 to a voltage/frequency converter 64.
The voltage/frequency converter 64 converts the angular position error APS (voltage) into a corresponding pulse signal whose frequency is proportional to the error signal applied. In the event that the antenna should rotate in a clockwise direction, a control signal CW is applied to a stepper motor driver 66. A stepper motor 68 responds by rotating the pedestal 22 (FIG. 2) in a clockwise direction. Similarly, if the error signal APE indicates that the antenna 40 should rotate in a counterclockwise direction, then the stepper motor driver 66 receives a control signal CCW and controls the motor 68 in a direction opposite to the above case (viz., counterclockwise direction). This loop control continues until the antenna angular position error reaches a zero value.
On the other hand, in the event that the antenna mounted vehicle enters the shadow of a large building (for example) and the satellite tracking is prevented, then the receive signal level RSL falls below the threshold. In such a case, the switch 58 allows the output of the rate gyro 60 to be applied to the voltage/frequency converter 64. Accordingly, the stepper motor driver 66 controls the motor 68 using the output of the rate gyro 60.
In order to accomplish precise tracking control, the rate gyro 60 is required to exhibit extremely high precision irrespective of the ambient conditions. However, such high precision rate gyros are very expensive and are required to be enclosed within a constant temperature chamber in order to ensure their accuracy. Further, it is inherently difficult to reduce the size of a high precision rate gyro and the maintenance of the same is both awkward and time consuming.